chore: update reference documentation linked from readme. (#2351)

As part of #2101, update reference documentation links in README to reflect current. Deleted 2.0.11 ref doc links as it's already out of support.
Added tag to allow auto update for the latest 4.x version ref doc and javadoc. However, for 3.x versions, manual update is still needed.

README.adoc

@@ -8,11 +8,12 @@ image:https://sonarcloud.io/api/project_badges/measure?project=GoogleCloudPlatfo
 This project makes it easy for Spring users to run their applications on Google Cloud.
 You can check our project website https://spring.io/projects/spring-cloud-gcp[here].
 
-For a deep dive into the project, refer to the Spring Framework on Google Cloud Reference documentation below or the https://googleapis.dev/java/spring-cloud-gcp/latest/index.html[latest Javadocs].
+For a deep dive into the project, refer to the Spring Framework on Google Cloud Reference documentation or Javadocs:
 
-* link:https://googlecloudplatform.github.io/spring-cloud-gcp/reference/html/index.html[Spring Framework on Google Cloud Latest]
-* link:https://googlecloudplatform.github.io/spring-cloud-gcp/3.4.0/reference/html/index.html[Spring Framework on Google Cloud 3.4.0]
-* link:https://googlecloudplatform.github.io/spring-cloud-gcp/2.0.11/reference/html/index.html[Spring Framework on Google Cloud 2.0.11]
+// {x-version-update-start:spring-cloud-gcp:released}
+* link:https://googlecloudplatform.github.io/spring-cloud-gcp/4.8.4/reference/html/index.html[Spring Framework on Google Cloud 4.8.4 (Latest)] - https://googleapis.dev/java/spring-cloud-gcp/4.8.4/index.html[Javadocs 4.8.4]
+// {x-version-update-end}
+* link:https://googlecloudplatform.github.io/spring-cloud-gcp/3.7.4/reference/html/index.html[Spring Framework on Google Cloud 3.7.4] - https://googleapis.dev/java/spring-cloud-gcp/3.7.4/index.html[Javadocs 3.7.4]
 
 If you prefer to learn by doing, try taking a look at the https://github.com/GoogleCloudPlatform/spring-cloud-gcp/tree/main/spring-cloud-gcp-samples[Spring Framework on Google Cloud sample applications] or the https://codelabs.developers.google.com/spring[Spring on Google Cloud codelabs].